The "Ogdoadic" monsters are all black monsters covered with golden plates and the higher the Level, the more "evolved" is their appearance, with "Nunu" and "Nauya" being basic serpentine creatures, "Flogos" and "Zohah" being bipedal and more akin to a lamia, respectively, "Aleirtt" and "Keurse" resembling beasts, "Aron" and "Amunessia" being more humanoid and holding a staff; and, finally, "Ogdoabyss" being a divine winged snake.
This archetype is based on the Egyptian Ogdoad (from which the TCG name comes from), eight primordial deities that personify the inherent qualities of the primeval waters of chaos. The eight deities are symbolically depicted as aquatic animals, such as frogs (masculine) and serpents (feminine). When the Ogdoad eventually converge together, it will result in a great upheaval that will produce the first benben, from which the sun will emerge to give light to all of creation.
The Ogdoad is divided into four male-female pairs, with each pair representing an aspect of the watery abyss: the stagnant water itself, the infinite expanse of its body, the darkness within its depth, and the invisibility of its incomprehensible nature. The "Ogdoadic" monsters are all depicted as serpentine, with the LIGHT-Attribute representing the masculine deities and the DARK-Attribute representing the feminine deities.
The archetype's Japanese name 「溟界」 (Meikai) is derived from the word 「冥界」 (Meikai, "Underworld"), but with the kanji 「冥」 ("Dark") replaced by 「溟」, which has the same pronunciation and similar meaning but a more oceanic connotation, evoking the feeling of "a world of darkness and water". Incidentally, the closest concept in modern Western thought, "abyss", is used in the Bible as translation of the Hebrew word tehom, which also refers to the primordial waters myth.
"Ogdoadic" is a Deck with a midrange/control-oriented approach that revolves around sending their monsters to the Graveyard and Special Summoning them afterwards. They are organized in pairs of DARK and LIGHT monsters with the same Level that have similarly-designed effects:
- The "Remnant" pair "Nunu"/"Nauya" are the starters of the Deck, each one having a self-discarding effect that sends an "Ogdoadic" monster of the opposite Attribute from the Deck to the Graveyard, while also filling the Graveyard for effects like those of "Ogdoadic Water Lily", "Ogdoadic Flood" and/or "Ogdoadic Calling" in the process. They are also extenders since "Nunu" can Special Summon itself, so it can be used for an Xyz/Link Summon or as fodder for a Tribute, and "Nauya" searches a Spell/Trap Card upon being Summoned.
- The first effect of each "Boundless" "Zohah" and "Flogos" synergyzes with "Aron" and "Amunessia", respectively. They can manipulate the opponent's cards by forcibly making them draw a card, send cards from their hand or field to the Graveyard, as well as Special Summon a monster from their Graveyard to their field, which can trigger the effects of those monsters. They can add themselves back to the hand, which can be used to reset their effects and/or manipulate the Graveyard.
- "Keurse" and "Aleirtt" require a Tribute in order to Special Summon themselves from the Graveyard and can also trigger the effects of "Aron" and "Amunessia" while generating card advantage; once their first effects have resolved, though, they have no more use besides serving as a wall or a beatstick. Instead, they can be used as a material for a Summon or Tributed right away so not only a more powerful "Ogdoadic" monster can replace them but their first effects will be reset for the next turn.
- The aforementioned "royal" couple "Aron" and "Amunessia" requires 2 Tributes each to Special Summon themselves and are the lynchpin of the Deck's control strategy, with effects that penalize the opponent for Special Summoning from the Graveyard or adding a card to their hand, which can be combined with the effects of the previous two pairs in order to disrupt the opponent's strategy by sniping cards they control or have in their hand; they also interact with each other, with the second effect of one triggering the third effect of the other, and were designed to stay on the field as long as possible so their effects can ensure not only a consistent, steadfast control of the game state, but also a continuous supply of Reptile monster from the Deck and Graveyard.
"Ogdoabyss, the Ogdoadic Overlord" can be considered the Deck's boss monster, as it has the highest Level and ATK, while also not having any on-theme counterpart so far. It requires 3 Tributes and once, as a Quick Effect, can send virtually all non-"Ogdoadic" monsters on the field to the Graveyard, which can be used to disrupt the opponent's strategy or clear the field for a finishing wave of attacks. Their Spell/Trap support is very versatile: "Ogdoadic Water Lily" and "Ogdoadic Calling" reward the player for having as many "Ogdoadic" monsters in the Graveyard as possible, Special Summoning Reptile monster(s) and potentially bypassing the Tribute costs of the "Ogdoadic" bosses. "Ogdoadic Flood" and "Ogdoadic Hollow" are Continuous Spell/Trap Cards with Main Phase effects that interact with the opponent's Graveyard in some degree and provide card advantage off the effects of "Aron" and "Amunessia" in the process, respectivelly. Finally, another noteworthy feature of the Deck is the mechanic of sending opponent's cards to the Graveyard, instead of destroying or discarding them, which is effective against monsters that cannot be destroyed by card effects (or that benefit from being destroyed), and also to avoid certain effects that activate if the monster is discarded.
"King of the Feral Imps" is an essential engine that not only searches for resources but also prevents "Nunu" from being banished after it has Special Summoned itself, as well as a Level 4 "Ogdoadic" revived by "Keurse". Also, being a Graveyard-reliant theme, "Snake Rain" is an indispensable card for the Deck's consistency. Other generic support cards for Reptile monsters, like "Damage = Reptile", or "Yamorimori" can also be included; "Lightserpent" can Special Summon itself after being sent to the Graveyard with the effects of "Zohah" or "Flogos", and then serve as fodder for Tributes. "Night Sword Serpent" is also an option to send to the Graveyard with "Snake Rain", "Ogdoadic Flood" or the second effect of "Zohah", as it will also Special Summon itself. Finally, "Danger!? Tsuchinoko?" can add a layer of consistency to the Deck by correcting the hand, setting up the Graveyard and giving a draw while also serving as a Tribute fodder. "Reptilianne" monsters in general also synergize well with the Deck, especially "Reptilianne Echidna", that can be used to search "Ogdoadic" monsters and/or "Reptilianne Lamia" and "Coatl", that can be used to Synchro Summon "Reptilianne Hydra", which in turn can singlehandedly provide impressive pluses in card advantage with the proper setup. "Aliens" can also be combined with the Deck, with "Alien Shocktrooper M-Frame" being capable of Summoning many "Ogdoadic" monsters from the Graveyard at once and "Cosmic Slicer Zer'oll" being useful to shut down any on-field activated monster effect the opponent may use.
Among non-Reptile options, "Lair of Darkness" can be used to alleviate the cost of Special Summoning the high Level "Ogdoadic" monsters by Tributing opposing monsters and/or Tokens, whilst "Darkest Diabolos, Lord of the Lair" can be used either as a Tribute fodder or for Rank 8 Xyz Summons. Also, being a LIGHT and DARK strategy, monsters like "Black Luster Soldier - Envoy of the Beginning" and/or "Chaos Dragon Levianeer" may also be useful. The Deck in general has little attachment to the Extra Deck; thus, "Pot of Extravagance" or "Pot of Prosperity" can be included to either provide the draw power or increase the odds of having access to fundamental cards like "Snake Rain".